There are many applications where it is necessary to protect electrical equipment from power surges, high energy transients and high frequency noise or random voltage embedded in a power sine wave that could damage or adversely affect the operation of such equipment. Such protection is particularly important for equipment having highly sensitive or complex loads that are susceptible to power surges and/or high frequency content transients. For example, conventional data processing equipment includes sensitive components which are particularly susceptible to damage or loss of stored data therein due to reactive voltage spikes occurring as a result of power surges in the supply lines, switching transients, or as a consequence of external causes such as lightning that strikes the supply line or produces an electromagnetic pulse which is inductively coupled into the supply line, or reactive spiking or ringing within the load's own power supply transformers which upsets the DC output of the internal power supply. Some of these conditions may also occur with respect to, and thus damage, sophisticated telecommunication and telephone equipment installations, for example.
Optimum surge suppression electronic filtering circuitry is designed to maintain a power waveform that, at all times, is the least disturbing to the load equipment it serves. Therefore, the high frequency content produced by an external power source, or by a surge suppressor, or by the load itself, must be kept to a minimum. A low suppression level close to the peak of a sine wave must also be obtained without distorting the source waveform and with maximum reliability of the suppressor.
These requirements rule out the use of many presently available suppressors. The fact that no single device adequately functions as a suppressor for all transients that may be encountered has led to the development of what are known in the art as hybrid circuits. Such hybrid circuits often combine a crowbar-type device such as a gas tube or the like, with a clamping device, such as an MOV (metal oxide varistor) and/or a selenium suppressor. Some of these suppressor circuits include several stages. However, these devices generally include circuit elements, such as inductors, in series in the power lines between the suppressor elements and/or suppressor stages. The present inventor has found that such series circuit elements reduce the effectiveness of such suppressors. For example, an inductor may limit current flow to a second stage thereby placing a high load on a prior stage. Such high loading may endanger one of the stages. Such series elements may also slow the overall reaction time of the suppressor, and may even make the reaction time so slow as to seriously vitiate the effectiveness of the suppressor. Still further, such series elements may influence the power wave applied to the electronic equipment being protected even if the suppressor has not been activated.
Still a further drawback to suppressors having a plurality of stages with circuit elements between the stages is the problem of placing the suppressor as close to the load as possible. As was discussed in U.S. Pat. No. 4,835,650, the disclosure of which is fully incorporated herein by reference, the leads in a surge suppressor network can influence the operation of that network. As is discussed in that patent, it is beneficial to shorten the lead lines in such networks as much as possible. Some surge suppression networks include a selenium element. Such elements are often too large to be physically placed close to the load or to other terminals. Therefore, there may be lead lines that may influence the operation of the suppressor network. This patent discussed this problem as being associated with let-though voltage.
Many presently available surge suppression networks place one of the elements thereof in an electrical location that may expose that particular element to high energy spikes. Such elements may fail thereby exposing the remainder of the network or the load to further problems. As discussed above, this is especially true in networks with series circuit elements interposed between stages of a multistage suppressor network.
Still further, many presently available protective networks have not been completely satisfactory for all conditions of service because they do not prevent the adverse effects of surges and transients occurring at the source, or as a consequence of local or internal circuit conditions such as a circuit breaker actuation, as well as being effective to prevent reactive spike build-up at the load side of the network. It is desirable to provide such surge and transient suppression in synergistic combination with power filtering, as well as to adapt such protective networks for convenient installation with multiple phase loads as well as single phase loads. Further, it is desirable that the protective network be capable of handling heavy loads so that it will not fail under high stress conditions. Many prior art approaches consider elementary spike protection only, not the overall protection and implications on the load equipment itself.
Other prior art approaches have attempted to parallel multiple avalanche diode, MOV or similar devices having identical V/I characteristics. In such applications, exacting testing must be performed to match all devices for identical performance. Such a match rarely can be maintained in use because of parts wear. Often, these devices still insert series circuit elements between stages. In some cases, the manufacturers of such devices may insert them merely to improve the way a specification sheet looks to a potential customer. However, these devices may fail in a very short time and will not be of real value.
Some prior hybrid circuits, such as disclosed in U.S. Pat. No. 4,616,286, include a selenium surge suppressor in parallel with an MOV and a capacitor. While somewhat effective, this type of circuit has drawbacks because the overall circuit is not designed as a unit but as discrete elements. For example, the just-mentioned patented circuit has the MOV element turn on after the voltage has exceeded the level of the selenium element turn-on voltage. This concept does not account for the action of the V/I curve of the overall circuit, and does not account for what happens to the current while the selenium device is conducting and before the MOV device has effectively activated. In this area of the V/I curve, substantial amounts of current can be conducted to the load, and may be damaging to some load elements under certain conditions. In fact, the just-mentioned patented device has discrete transition levels for each element. This concept may have "discontinuities" in the V/I curve associated with transition in operating points for the individual elements of the overall network that may not be desirable in some situations. In such networks, the current associated with the load is not precisely controllable when the selenium device is turned on and the MOV has not yet reached the clamping state. Such current can be significant in some situations and may damage suppressor elements such as the selenium or MOV.
A still further problem with prior art hybrid networks such as disclosed in the U.S. Pat. No. 4,616,286 patent results because the resistance of the lead lines connecting the single MOV to the power leads associated with the load may be significant with respect to the resistance of the single MOV when that MOV turns on whereby the overall electrical characteristics of the protector network with respect to the load may be adversely affected. However, since devices such as this patented device are simply using the MOV as a "step-in" element such problems are not considered.
Yet a further problem with many prior art networks is the difficulty and expense associated with replacing one of the circuit elements that may have failed. This problem is exacerbated by the set up of many prior art networks that expose one or more of the elements thereof to excessively high energy spikes.
Therefore, there is a need for a power surge and transient voltage protection network that reacts quickly enough to be able to accommodate modern electronic equipment, and will have a minimum let-through voltage, while not influencing the wave form of the power being applied to the load or placing any element of the network in a position to be exposed to an unduly high level of an energy spike. More specifically, there is a need for a power surge and transient voltage protection network that can have the overall V/I characteristics thereof customized to meet the exact needs of the particular load, and can be modified so current flow in the power conductor lines can be controlled at all times during activation of the network, including the time when one suppressor element is turned on but the voltage across the load lines has not yet reached a level associated with the turn on clamping voltage of a second suppressor element.
The parent application discloses and discusses a panel that can be used to connect electronic equipment to a source of power in a manner that permits efficient and accurate connection of power surge protecting and filtering circuits into a building wiring system. This panel has protection circuitry installed therein to provide voltage and transient protection. While this panel is extremely effective, it can be further improved by combining it with a power surge and transient protection network having the V/I characteristics thereof customized as discussed above. In such a set up, the V/I characteristics of the overall network can be further controlled using the set-up disclosed herein in combination with elements located directly in the panel. In this manner, still further customizing can be achieved.
Specifically, the inventor has noted that conductors connecting elements or circuits in a system together or to other elements can cause losses in voltage in the overall system. To be most effective, such line losses should be accounted for in surge and transient protection systems. If such line losses are properly accounted for, the overall V/I characteristics of a system can be quite accurately customized.
For maximum efficiency, the customizing should be amendable to easy mechanical connection and efficient integration into a standard electrical distribution system.